#117 Warning about MJML missing when using MRML

[sighmon]

Resolves #117

• [x] Add a check for a valid MRML binary

Testing

1. Uninstall/remove mjml from your environment
2. Clone and switch to using this branch in your Gemfile: gem 'mjml-rails', path: '../mjml-rails/'
3. Add config.use_mrml = true to your initializer
4. Add mrml to your Gemfile
5. Note the warning of a missing MJML binary no longer occurs